It would act as an equalizer

I agree government forms should come with glossaries. I live in Canada and In 2012, 51.5% of Canadians aged 16 to 65 had literacy scores in the Level 3 category or above. Which leaves 48.5 % of the population that have a literacy score under level 3 which keep in mind that the scale stops at level 5. That is a fairly big amount to expect and assume those individuals in the lower end of the bracket to understand the meaning of every word. My theory is it would act as an equalizer for the individuals that are not well informed or educated strictly speaking on Literacy skills.

It would make it easier to fill out

Current practice is they just assume that whoever is filling out the form knows the definition of every word in the form, including ones that may not apply to their particular case (but where you wouldn't realize that unless you knew the definition of the word).

Why not include a glossary in government forms, so people can look in the glossary and figure out what the obscure, ambiguous words mean?